The method of applying a vapor retarder sheet or the like as one step of applying a roofing system is described in full detail in U.S. Pat. No. 4,736,552, which issued on Apr. 12, 1988 to Lonnie R. and Cherryl A. Ward, and that patent is incorporated by reference as if fully recited herein. The Ward '552 patent teaches a roof system and method of fabricating, as used on a building with a plurality of parallel and spaced apart elongated structural support units defining a longitudinal surface exposed from above. The spacing between these structural support units is generally consistent across the longitudinal surface. A first step in applying a roof to this type of structure involves securing a flexible, generally moisture impervious sheet over the longitudinal surface to act as a vapor retarder between the building interior and exterior. The method as described in Ward '552 involves placement of a double-faced adhesive tape along the top surface of the support units and then applying lengths of the sheet material as payed off of rolls thereof atop the doubled faced adhesive tape which are on adjacent support units. The sheet material applied is sufficiently wide to cover the adjacent top surfaces and the intervening space therebetween. By applying a second piece of the double-faced adhesive tape over a vapor retarder sheet atop the first piece of double-faced adhesive tape, an adhesive surface for placement of an overlapping sheet of the vapor retarder material is provided. In this manner, a continuous uninterrupted surface of the vapor retarder material is secured to the top surface of the support units. Although the Ward '552 patent teaches the method of applying such a roofing system and also the roofing system achieved as a result of the method, it does not teach a device for accomplishing the step of adhering the vapor retarder to the roof supports.